The ornamental design for an atomizer, as shown and described.
FIG. 1 is a side view of an atomizer;
FIG. 2 is a top view of the atomizer depicted in FIG. 1;
FIG. 3 is a side view of the of the atomizer depicted in FIG. 1, shown with the bottom portion removed to show aspects of the design not visible when combined; and,
FIG. 4 is an opposite side view of the of the atomizer depicted in FIG. 3.
The broken lines in the figures represent portions of the article which form no part of the claimed design.